In magnetic disk drive systems, data is written to and read from magnetic disks contained therein. Typically, the disk is mounted on a spindle such that the disk can rotate to permit an electromagnetic head mounted on a movable arm positioned near the disk's surface to read or write information on the disk.
During operation, the system moves the head to a desired radial position on the surface of the rotating disk where the head electromagnetically reads or writes data. Usually, the head is integral part of a carrier called a "slider". A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the drive system.
The slider is aerodynamically shaped to glide over moving air, so that it generally maintains a uniform distance from the surface of the rotating disk and does not undesirably contact the disk.
A slider is typically formed with two parallel rails and a recessed area between the rails. The surface of each rail that glides over the disk surface during operation is known as the "air-bearing surface". The head contains magnetic poles that terminate at the air bearing surface; these poles function to interact with the magnetically-recorded information on the disk during operation. The tip portions of the poles are parallel and separated by a magnetically insulating region called the "throat". The length of this throat region, which is commonly referred to as the "throat height" of the head, must be exact to within very strict tolerances in order to achieve maximum performance of the head. Accordingly, thin-film head fabrication techniques generally provide for precise control of final throat height.
Thin-film sliders are commonly fabricated from a single wafer upon which an array of transducers is formed using known wafer-processing techniques. After formation of the transducers, their pole portions and throat regions lie in the plane of the wafer. During subsequent processing steps, individual sliders are separated from the wafer, exposing the air-bearing surface of each slider that lies in a plane perpendicular to the plane of the wafer. The pole portions of each transducer terminates at the air-bearing surface of the corresponding slider. The air-bearing surface is then finely lapped to bring the throat height to the proper value.
It is common to arrange the sliders in parallel rows upon the wafer, and to subsequently slice the wafer into separate rows for further machining. For example, each sliced row can be temporarily bonded to a rigid carrier tool in preparation for lapping. This row-tool combination can then be held in a fixture during the lapping process, and upon completion the row can be sliced into individual sliders and debonded from the tool. It is also common to incorporate features known as "lapping guides" on each row that are monitored during the lapping process to determine when lapping should be terminated. These lapping guides are typically electrical circuits whose behavior changes as lapping progresses. For example, the lapping guides may incorporate a set of switches that open progressively as material is removed.
While the above-described fabrication technique is fairly simple, it suffers from a serious problem that is commonly referred to as "row bow" or simply "bow". If the row being lapped is not flat, then the sliders in a row will not be lapped by the same amount. As a result, the throat heights of the sliders in the row will deviate substantially from the desired throat height. If the bow is sufficiently severe, this uneven lapping will result in poor yield, because many of the resulting heads will either not function or will fail to meet operating specifications. The problem of row bow, then, must be accounted for in thin-film head fabrication processes.
It should be noted that there are other factors that contribute to final throat height deviation. For example, imperfections in the mechanics of lapping machinery introduce deviation at the lapping stage. Also, imprecise lap guide positioning with respect to the plane of the throats during wafer deposition can also cause throat height errors. However, row bow as described has been the dominant factor, and it this problem in particular that the present invention addresses.
Several approaches to eliminating or reducing row bow are shown in U.S. Pat. No. 5,095,613, entitled "Thin Film Head Slider Fabrication Process", by Hussinger et al., issued Mar. 17, 1992 and assigned to Digital Equipment Corporation. For example, in the process of FIG. 3 of that patent, each row of sliders is lapped while still attached to the wafer. This of course provides for great accuracy, because the rows can be formed very straight on the wafer, and it is practically impossible for "bow" to occur in the plane of the wafer. In the process of FIG. 7, a rigid carrier tool is bonded to a row while it is still part of the wafer, and then the row is sliced off. This technique ensures that the row is always backed by a straight, rigid object, so that little or no bow can be introduced therein. Finally, FIGS. 9 through 11 show techniques wherein the wafer is sliced into 2-row bars, the rows in each bar facing either toward or away from each other. These 2-row bars are described as being substantially stiffer than single-row bars, so that they are less likely to bow. The 2-row bars are bonded to rigid carriers, sliced, and lapped.
While the techniques shown in the Hussinger patent clearly reduce harmful rowbow, they also have some practical drawbacks or limitations. For example, the processes of FIGS. 3 and 7 require that an entire wafer be used during the slicing and lapping processes. The portion of the wafer that is not being processed, which of course is all of the wafer except for one row, constitutes excessive "work in progress" or WIP. This term refers to the amount of intermediate work material that must be created before finished material can result. The larger the amount of WIP, the less efficient the fabrication process is. Generally, as the amount of WIP increases, the overall time from starting a wafer to packaging a finished head increases. Also, when process defects are detected, especially those occurring in the beginning process stages such as wafer fabrication, much or all of the WIP must be scrapped. It is therefore generally desirable to minimize the amount of WIP in a fabrication process. But the processes of FIGS. 3 and 7 of Hussinger actually increase the amount of WIP over more conventional thin-film head fabrication techniques.
Hussinger teaches, in connection with the processes of FIGS. 9-11, that 2-row bars are substantially stiffer than single-row bars, and then shows two distinct ways of processing 2-row bars. In one method, the rows are formed on the wafer to face each other. Two rigid carrier tools are attached to the outside surfaces, then the rows are split apart and lapped separately. In the other method, the rows are formed to face away from each other. Then a somewhat complex series of tool bonding/debonding, slicing, and lapping steps is performed.
Both of these techniques of Hussinger rely on an unusual row orientation, where alternating rows face in opposite directions. The first technique can apparently be used only with 2-row bars wherein the rows face each other, and the second technique similarly relies on having only 2 rows in a bar. Hussinger teaches that having 2 rows in a bar is sufficient to overcome bow, and mentions no conditions under which this might not be true. The limitations in Hussinger's processes are therefore consistent with Hussinger's view that slicing the wafer into 2-row bars is a general solution to the bowing problem.